There is an increasing interest in reducing power consumption in communication systems, particularly the power consumption of base stations in cellular radio telephone systems. Reducing power consumption on the user side of a system, i.e., mobile phones, portable computers, and other user equipments (UEs), has been a long-time effort, not least because UEs are often battery-powered. The desire for improved energy efficiency on the network side, i.e., base stations and other network nodes, is mainly driven by the cost to the network operator of transmitting “unnecessary” signals in cases of low network load.
Communication systems that can have “unnecessary” signals are, among others, systems that conform to the High Speed Packet Access (HSPA) and Long Term Evolution (LTE) telecommunication standards. HSPA systems and LTE systems, which can include HSPA, are sometimes called “third generation” (3G) cellular communication systems and are currently being standardized by the Third Generation Partnership Project (3GPP). The LTE specifications can be seen as an evolution of the current wideband code division multiple access (WCDMA) specifications. An IMT-Advanced communication system (i.e., a “fourth generation” (4G) system) uses an internet protocol (IP) multimedia subsystem (IMS) of an LTE, HSPA, or other communication system for IMS multimedia telephony (IMT). The 3GPP promulgates the LTE, HSPA, WCDMA, and IMT specifications, and specifications that standardize other kinds of cellular wireless communication systems.
In an LTE system, an enhanced Node B (eNB), or base station (BS), can be configured for downlink (DL) channels that have bandwidths ranging from about 1.4 megahertz (MHz) to 20 MHz for communicating with UEs. Such channels are supported by pilot signals having standardized patterns corresponding to the channel bandwidth, with lower-bandwidth channels requiring transmission of fewer pilot signals than higher-bandwidth channels. It is possible for an eNB to be set for 20-MHz bandwidth channels at times when that bandwidth is not needed, e.g., because there are not enough UE-requests for data transmissions. Thus, despite not all DL resource blocks (RBs) being populated with data, the standardized but unnecessary pilot signal pattern is transmitted, wasting energy in the eNB, and UEs patiently do their jobs reporting unnecessary channel quality indicators (CQIs) for large numbers of unused resource blocks, wasting energy in the UEs.
In an HSPA system, a BS can be configured for transmit (TX) diversity and so have two active transmitters with respective pilot signal patterns set up for HSPA multiple-input multiple-output (MIMO) dual stream transmission. Nevertheless, such operation may be unnecessary because there may not be any MIMO-capable UEs in the vicinity of the BS. There may not even be any nearby UEs capable of receiving a TX diversity scheme and/or the UEs that are nearby may not be requesting much downlink data at that moment. Thus, a BS configured for TX diversity wastes energy transmitting unnecessary pilot signals by its second transmitter when it is configured for high-speed data while such high data speeds are not used. In addition, the UEs in the vicinity of a TX-diversity BS may be of a less capable kind or even have trouble processing the TX-diversity pilot signals properly, and so those UEs may waste battery energy on processing the more complicated TX-diversity pilot signals and/or may suffer decreased system performance.
FIG. 1 depicts a typical cellular radio communication system 10. Radio network controllers (RNCs) 12, 14 control various radio network functions, including for example radio access bearer setup, diversity handover, etc. In general, each RNC directs calls to and from a UE, such as a mobile station (MS), mobile phone, or other remote terminal, via appropriate BSs, which communicate with each other through DL (or forward) and uplink (UL, or reverse) channels. In FIG. 1, RNC 12 is shown coupled to BSs 16, 18, 20, and RNC 14 is shown coupled to BSs 22, 24, 26.
Each BS, or eNB in an LTE system, serves a geographical area that is divided into one or more cell(s). In FIG. 1, BS 26 is shown as having five antenna sectors S1-S5, which can be said to make up the cell of the BS 26, although a sector or other area served by signals from a BS can also be called a cell. As described above, a BS can use more than one antenna to transmit signals to a UE. The BSs are typically coupled to their corresponding RNCs by dedicated telephone lines, optical fiber links, microwave links, etc. The RNCs 12, 14 are connected with external networks such as the public switched telephone network (PSTN), the internet, etc. through one or more core network nodes, such as a mobile switching center (not shown) and/or a packet radio service node (not shown).
It should be understood that the arrangement of functionalities depicted in FIG. 1 can be modified in LTE and other communication systems. For example, the functionality of the RNCs 12, 14 can be moved to the eNBs 22, 24, 26, which is typically so in an LTE system, and other functionalities can be moved to other nodes in the network. For example according to a System Architecture Evolution (SAE) being standardized by 3GPP, the eNBs 22, 24, 26 communicate with an SAE gateway node in the core network via an S1 interface, with the eNBs and SAE gateway comprising a user plane of the SAE architecture.
The LTE physical layer, including the physical downlink shared channel (PDSCH) and other LTE channels, is described in 3GPP Technical Specification (TS) 36.211 V8.7.0, Physical Channels and Modulation (Release 8) (June 2009), among other specifications. LTE communication systems are described in the literature, for example, U.S. Patent Application Publication No. US 2008/0031368 A1 by B. Lindoff et al. For cell measurements, channel estimation, and other purposes, pilot signals, or reference symbols or signals (RS), are transmitted from each eNB at known frequencies and time instants. RS are described for example in Sections 6.10 and 6.11 of 3GPP TS 36.211, and are transmitted from each of possibly 1, 2, or 4 transmit antennas of an eNB on particular resource elements (REs). Comparable specifications and literature are available for WCDMA and other communication systems.
FIG. 2 shows an arrangement of subcarriers in RBs in two successive time slots, which can be called a sub-frame, in an LTE system. Like many modern digital communication systems, LTE and HSPA signals are organized in frames, and the length of an LTE frame is twenty slots. The frequency range depicted in FIG. 2 includes twenty-seven subcarriers, only nine of which are explicitly indicated. In FIG. 2, the RBs, which are indicated by dashed lines, each include twelve subcarriers spaced apart by fifteen kilohertz (kHz), which together occupy 180 kHz in frequency and 0.5 ms in time, or one time slot. FIG. 2 is scaled such that it shows each time slot including seven symbols, or REs, each of which has a short (normal) cyclic prefix, although six OFDM symbols having long (extended) cyclic prefixes can be used instead in a time slot. It will be understood that RBs can include various numbers of subcarriers for various periods of time.
RS transmitted by a first TX antenna of an eNB are denoted R and by a possible second TX antenna in the node are denoted by S. RS are transmitted on every sixth subcarrier in OFDM symbol 0 and OFDM symbol 4 (because the symbols have short cyclic prefixes) in every slot. Also in FIG. 2, the RSs in symbols 4 are offset by three subcarriers relative to the RS in OFDM symbol 0, the first OFDM symbol in a slot.
Besides reference signals, predetermined synchronization signals are provided for a cell search procedure that is a UE carries out in order to access the system, or network. The cell search procedure includes synchronizing the UE's receiver with the frequency, symbol timing, and frame timing of a cell's transmitted signal, and determining the cell's physical layer cell ID. The cell search procedure for an LTE system is specified in, for example, Section 4.1 of 3GPP TS 36.213 V8.6.0, Evolved Universal Terrestrial Radio Access (E-UTRA), Physical Layer Procedures (Release 8), June 2009. LTE uses a hierarchical cell search scheme similar to WCDMA, in which eNB-UE synchronization and a cell group identity (ID) are obtained from different synchronization channel (SCH) signals. A primary synchronization signal (PSS) and a secondary synchronization signal (SSS) are defined with a pre-defined structure in Section 6.11 of 3GPP TS 36.211.
FIG. 2 shows the SSS and PSS as OFDM symbols 5, 6 (assuming operation with the short cyclic prefix and frequency-division duplex (FDD). Current LTE systems have the PSS and SSS symbols transmitted in the middle six RBs (i.e., the middle seventy-two subcarriers) in sub-frames 0 and 5. In general, a UE uses the PSS to synchronize to the slots and the SSS to synchronize to the frames in an LTE system. Comparable reference and synchronization channels are often provided in other digital communication systems, although they may be given different names.
As discussed above, a BS can have a transmission/reception setup that is not currently necessary and so it is desirable to improve the efficiency of BS operation.